The primary objective of the proposed research is to elucidate the mechanisms of visual excitation and adaptation. Our approach is to modify the intracellular and extracellular millieu of the photoreceptor in order to investigate the different processes involved in transduction. We have developed a method using microspectrophotometry for monitoring intracellular pH and Ca while simultaneously monitoring sensitivity. The experiments proposed will yield an improved understanding of the molecular changes underlying visual transduction.